Tomofumi Susaki

New functionalities in abundant element oxides: ubiquitous element strategy

Abstract While most ceramics are composed of ubiquitous elements (the ten most abundant elements within the Earths crust), many advanced materials are based on rare elements. A ‘rare-element crisis’ is approaching owing to the imbalance between the limited supply of rare elements and the increasing demand. Therefore, we propose a ‘ubiquitous element strategy’ for materials research, which aims to apply abundant elements in a variety of innovative applications. Creation of innovative oxide materials and devices based on conventional ceramics is one specific challenge. This review describes the concept of ubiquitous element strategy and gives some highlights of our recent research on the synthesis of electronic, thermionic and structural materials using ubiquitous elements.

Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface

A d-orbital electron has an anisotropic electron orbital and is a source of magnetism. The realization of a two-dimensional electron gas (2DEG) embedded at a LaAlO3/SrTiO3 interface surprised researchers in materials and physical sciences because the 2DEG consists of 3d-electrons of Ti with extraordinarily large carrier mobility, even in the insulating oxide heterostructure. To date, a wide variety of physical phenomena, such as ferromagnetism and the quantum Hall effect, have been discovered in this 2DEG system, demonstrating the ability of d-electron 2DEG systems to provide a material platform for the study of interesting physics. However, because of both ferromagnetism and the Rashba field, long-range spin transport and the exploitation of spintronics functions have been believed difficult to implement in d-electron 2DEG systems. Here, we report the experimental demonstration of room-temperature spin transport in a d-electron-based 2DEG at a LaAlO3/SrTiO3 interface, where the spin relaxation length is about 300u2009nm. Our finding, which counters the conventional understandings of d-electron 2DEGs, highlights the spin-functionality of conductive oxide systems and opens the field of d-electron spintronics.

Fe3O4(1 1 1) thin films with bulk-like properties: growth and atomic characterization

Growth defects, domain structures and off-stoichiometric compositional deviation are the cause of significant deviation of magnetic and transport properties of magnetite (Fe3O4) thin films. We show that these anomalous properties can be fully controlled by postannealing in CO/CO2 atmosphere. Aberration corrected electron microscopy has revealed a full structural transformation from Fe/FeOx to Fe3O4 and a drastic reduction of growth defects in the postannealed films. The magnetic and magnetotransport measurements show that postannealed film properties are bulk-like: the magnetization was fully saturated with ∼480xa0emuxa0cm−3, as in bulk single crystal, and the negative magnetoresistance was suppressed by an order of magnitude.

Controlled bipolar doping in Cu3N (100) thin films

We have fabricated insulating, p- and n-type Cu3N(100) films on SrTiO3(100) by plasma assisted molecular beam epitaxy. By controlling the Cu/N flux rate, p-type doping with 1018–1020u2009cm−3 in Cu-poor condition and n-type doping with 1019–1020u2009cm−3 in N-poor condition were obtained without introducing foreign species. Together with formation of insulating Cu3N films with an optical absorption coefficient of ∼105u2009cm−1 in the photon energy above ∼2.2u2009eV and an estimated indirect bandgap of ∼1.3u2009eV, the bipolar doping in Cu3N films would be promising for solar energy conversion applications.

Structural study of Fe3O4(111) thin films with bulk like magnetic and magnetotransport behaviour

Post-annealing of Fe3O4 films in a CO/CO2 atmosphere results in a significant improvement in magnetic and magnetotransport properties with values close to the single crystal bulk of Msu2009∼u2009480u2009emu/cm3 and negative magnetoresistance of 0.05% in a field of 1u2009T. By using atomic resolution Z-contrast transmission electron microscopy, we show that improved magnetic properties in the annealed films are due to improved structural ordering as a result of the annealing process.

Tuning of Surface Roughness and Lattice Constant in MgO(111)/Al2O3(0001) Grown by Laser Energy Controlled Pulsed Laser Deposition

We have studied the effect of variable laser energy on pulsed laser deposition (PLD) grown MgO(111) films on Al2O3(0001) substrates by X-ray diffraction and reflectivity measurements, atomic force microscopy and reflection high energy electron diffraction. In spite of the polar instability of MgO(111) surface, the surface roughness drastically decreases from ~2 to ~0.5 nm as the laser energy increases from ~50 to ~150 mJ. The in-plane lattice constant is larger than the bulk value at the smaller laser energy while it is smaller than the bulk value at the larger laser energy. This indicates that the balance between the electrostatic energy due to the polar structure, which favors a larger in-plane lattice constant, and the interface epitaxial strain, which favors a smaller in-plane lattice constant in MgO(111)/Al2O3(0001), can be tuned by varying the laser energy in PLD.

Magnetotransport Properties across Verwey Transition in Fe3O4(111) Epitaxial Thin Films

We have studied the magnetoresistance (MR) of high-quality Fe3O4 thin films across the Verwey transition temperature (TV). Different from unsaturated MR observed in many other Fe3O4 films, MR of the present film at a high magnetic field is almost independent of the field, consistent with the fully saturated magnetization. The MR at low field strongly depends on temperature and orientation around TV: While out-of-plane MR sharply decreases from ~+7 to ~-5%, in-plane MR abruptly increases from ~-6 to ~+3% on cooling across TV.

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains.

Examination of interfacial charge transfer in photocatalysis using patterned CuO thin film deposited on TiO2

We examined the interfacial charge transfer effect on photocatalysts using a patterned CuO thin film deposited on a rutile TiO2 (110) substrate. Photocatalytic activity was visualized based on the formation of metal Ag particles resulting from the photoreduction of Ag+ ions under visible-light illumination. Ag particles were selectively deposited near the edge of CuO film for several nanometer thick CuO film, indicating that interfacial excitation from the valence band maximum of TiO2 to the conduction band minimum of CuO plays a key role in efficient photocatalytic activity of CuO nanocluster-grafted TiO2 systems with visible-light sensitivity.

Control of Surface Work Function by Depositing Insulating Oxide Capping Layers

We have studied the work function modulation in pulsed laser deposited nonpolar MgO (100) and polar MgO (100) and LaAlO3 (100) insulating oxide films on Nb:SrTiO3 (100) and (111) substrates using Kelvin probe measurements. The work function modulation in MgO/Nb:SrTiO3 is ~1 eV, which may be understood by the electron compression model, where the leaked-out electrons from Nb:SrTiO3 surface are pushed back by MgO layer, reducing the surface dipole moment. By forming double insulating top layer in LaAlO3/nondoped SrTiO3/Nb:SrTiO3 (100), we have realized the work function as low as ~2.2 eV, which would be originated in the formation of high-density electrons at the LaAlO3/nondoped SrTiO3 interface and can be very close to the top surface. The present results demonstrate that chemically stable oxides can be used as low work function devices employing heterostructure form.